1. Field of the Invention
This invention broadly relates to methods and apparatus for affixing brackets to the surfaces of teeth as used in orthodontic treatment. More specifically, the invention relates to a novel orthodontic bracket design and the associated bonding methods that provide enhanced clinical convenience and bond strength between the bracket and the tooth.
2. Description of Related Art
Orthodontic treatment involves moving teeth to improve occlusion. Current practice involves cementing slotted orthodontic appliances, known as brackets, to the teeth. To effect movement, an arch wire is inserted into the slot of each bracket. The wire is typically held in the slot by small elastics or wire ligature. The arch wire then exerts appropriate force via the brackets, to move the teeth and effect treatment.
Orthodontic brackets are designed to be bonded directly to the surface of patient's teeth using a bonding adhesive. The adhesives most commonly used are light-curable, and begin to set once light of the appropriate wavelength and intensity is directed onto the adhesive. Light-curable adhesives are generally preferred by orthodontists, as compared to chemically cured adhesives, mainly because the bracket can be readily repositioned prior to fixing it in place using the light.
Current bonding practice involves placing adhesive on the rear face of the bracket base and placing the bracket on a tooth. Some brackets are supplied with preapplied adhesive. The viscous nature of the adhesive causes the bracket to ‘stick’ to the tooth, allowing it to be repositioned by the orthodontist. A hand-held curing light, typically emitting blue visible light, is then directed along two or more edges of the bracket base to cure the adhesive between the back of the bracket and the tooth. Several practical clinical factors contribute to poor and variable bonding.
Since the curing radiation cannot pass through metal brackets, the adhesive under the bracket base must cure by conducting light from the perimeter of the base through the thin layer of adhesive between the bracket base and the tooth, and by reflecting through the tooth structure. This poor curing mechanism may not sufficiently react the adhesive in the central area of the bracket which can lead to debonding of the bracket when the bracket is subjected to a sufficiently large force.
Other factors contribute to a non-uniform cure of brackets. The orthodontist may typically place all the brackets in a quadrant (about 5) or an entire arch (about 10) prior to curing. In the time from initial placement to actual curing, the bracket may sag slightly or be accidentally touched and moved while placing the other brackets.
To achieve maximum bond strength, the thickness of the adhesive should be kept to a minimum. Pushing the bracket base against the tooth during bonding is the ideal way to both express excess adhesive and ensure intimate contact between the tooth and the bracket. Use of an external curing light prevents the clinician from ensuring intimate contact of the bracket during the curing process.
The intensity of the light that actually impinges on the adhesive depends upon many factors including the distance from the curing light to the bracket edge. While the tip of the light is typically held as closely as possible to the bracket, there is some natural variation in distance. Requiring the light tip to be held closely also poses the risk of touching a bracket, which would move the bracket from its desired position. A significant source of bonding variation is associated with the difficulty accessing all of the edgres of brackets bonded to rear teeth. In addition, variations exist in the type and radiant intensity of commercial curing units, as well as the clinical exposure times used by the orthodontist.
All of these factors affect the quality of cure and bracket adhesion. The clinician wants bracket bonding to be quick, have complete cures, maximum strength, and be consistent. Once the desired location is identified, placement and curing should proceed quickly and consistently.
A variety of approaches have been developed to improve the bonding of orthodontic brackets. Approaches have included new adhesives, priming agents, bracket designs as well as curing lights. Some brackets use bases with undercut regions, roughened surfaces, spherical particles, and fine metal mesh.
One of the more common modifications involves the texture and geometry of the bonding surface of the bracket to enhance the mechanical locking of the adhesive. For example, U.S. Pat. No. 5,267,855 describes the use of partially embedded textured particles on the bonding base and U.S. Pat. No. 5,480,301 teaches the use of a metal mesh, a layer of metallic or plastic particles, grooves and undercuts. U.S. Pat. No. 5,722,826 describes the use of various metal meshes bonded to a photo-etched metal foil backing, as well as sandblasting and even ion bombardment.
One of the earliest proposed design enhancements involves expressing some of the adhesive through the bracket pad to enhance retention. U.S. Pat. Nos. 3,932,940, 4,094,068 and 5,435,720 all describe substantially the same approach, wherein brackets are provided with holes in the base to allow some adhesive to seep through and provide a better lock to the bracket.
Other approaches to improve bracket bonding are based upon variations in the curing light system. Several approaches include the use of light-conducting arch-shaped trays that allow an entire quadrant or arch of positioned brackets to be simultaneously illuminated and bonded. U.S. Pat. Nos. 5,316,473 and 5,813,854 describe such designs which still require an external light source and have the potential for contacting and moving the bracket.
An approach described by Rueggeberg in U.S. Pat. No. 5,800,163, describes a tip to be placed at the end of a curing light assembly. The tip directs the light as a 360° ring towards the edge of the bracket base parallel to the tooth surface. This design is meant to address the problem of accessibility to the bracket edges when bonding to rear teeth, curing all around the perimeter of the bracket pad. The method has the drawbacks of still requiring an external light source and the possibility of physically contacting adjacent brackets by the relatively wide tip design.
Another bracket design approach is found in U.S. Pat. Nos. 5,711,665 and 6,482,002. This orthodontic bracket design allows curing light to be directed normal to the bracket face instead of around the edges. A clear light guide in the body of the bracket is provided that allows curing light to be transmitted through the bracket body and onto the rear bonding surface. This design still requires the use of an external light source, but does begin to address the issue of directing curing light onto the rear bonding face of the bracket instead of the edge.
There is a continuing need to improve the strength, consistency, and efficiency of bonding orthodontic brackets to the patient's teeth to minimize the debonding of brackets during treatment. Premature debonding of orthodontic brackets represents a nuisance to the orthodontist and the patient, since the patient must return to the orthodontist for rebonding or replacement of the detached bracket to resume treatment.